(1) Field of the Invention
The present invention relates to a protection circuit for protecting a semiconductor chip from damage due to electrostatic charges.
(2) Description of the Prior Art
Electrostatic charge generated by friction between materials having different dielectric constants can cause fetal damage to a semiconductor device by causing oxide breakdown or internal discharge in the thin oxide layer of the semiconductor. Therefore, when designing a semiconductor chip, a protection circuit is provided so that the internal circuit of the semiconductor chip will not be damaged by electrostatic charges. Particularly, to keep pace with the increasing circuit density and performance of semiconductor chips as a result of advancements in manufacturing techniques, it is necessary to provide a circuit to protect the chip from the electrostatic charges. Recently, such a protection circuit has become an important factor in the reliability of the chip.
This protection circuit has discharge paths formed between an input pin and an output pin, and between a power terminal and a ground terminal such that electrostatic charges will be discharged through these paths. Thus, the electrostatic charges will not be lead to the internal circuit.
The internal circuit includes various devices, such as resistors, diodes, punch-through transistors, field effect transistors, and parasitic capacitors. Among these devices, the diode and the transistor provide a discharge path for electrostatic charges, while a resistor is connected between the input pin and the input terminal of the internal circuit to determine the level of the input current. Further, between the output terminal of the internal circuit and the output pin, there can be disposed a resistor, depending on the capability of the output drive, but in most chips, this is not used.
An example of such a protection circuit is illustrated in FIG. 5. This circuit has a diode D.sub.1 connected between an input pin 1 and a power terminal V.sub.DD. The output pin 1 is connected through a resistance R.sub.1 to an input terminal A of an internal circuit 2. Diodes D.sub.2, D.sub.3 are respectively connected between the input terminal A and the power terminal V.sub.DD, and between the input terminal A and ground G. An output terminal B of the internal circuit 2 is connected to an output pin 3. Diodes D.sub.4, D.sub.5 are connected respectively between the output terminal and power terminal V.sub.DD, and between the output terminal and ground G. A diode D.sub.6 is connected between the power terminal V.sub.DD and the ground G.
In such a protection circuit, on the one hand, if positive electrostatic charges are supplied between the input pin 1 and power terminal V.sub.DD, the positive electrostatic charges are discharged through the diodes D.sub.1, D.sub.2, and therefore, the electrostatic charges are not applied to the internal circuit 2. Here, resistor R.sub.1 is in the form of a polysilicon- and a diffusion-type resistance, thereby setting the level of current flowing to the diode D.sub.2.
On the other hand, if negative electrostatic charges are supplied between the input pin 1 and power terminal V.sub.DD, a discharging path is formed in the reverse direction of the diodes D.sub.1, D.sub.2. Under this condition, the current level is decided at the reverse direction breakdown region of the diodes, and therefore, the diodes D.sub.1, D.sub.2 can be damaged by even a low level of electrostatic charges to such a degree that the characteristics of the diodes are degraded, thereby reducing the reliability of the protection circuit.
In the case where electrostatic charges are supplied between the input pin 1 and ground terminal G, if the electrostatic charges are negative, the diode D.sub.3 discharges the electrostatic charges in the forward bias state. By contrast, if the electrostatic charges are negative, the diode D.sub.3 is damaged in the manner described above.
Further, in the case where electrostatic charges are supplied between the power terminal V.sub.DD and ground terminal G, if the charges are positive, the diode D.sub.6 smoothly discharges them in the forward bias state, while, if negative, the diode D.sub.6 forms a discharge path in the reverse direction, and so the diode D.sub.6 is likely to be damaged.
Where electrostatic charges are supplied between the output pin 3 and power terminal V.sub.DD, and between the output pin 3 and ground terminal G, the diodes D.sub.4, D.sub.5 form discharge paths in a selective manner. However, under this condition, there are cases where the diodes D.sub.4, D.sub.5 form discharge paths in the reverse direction, thereby damaging the diodes.
FIG. 6 illustrates another protection circuit using transistors M.sub.1, M.sub.2, M.sub.3, M.sub.4, instead of the diodes D.sub.2, D.sub.3, D.sub.4, D.sub.5 of FIG. 5. In this circuit, the channel regions of the transistors M.sub.1 to M.sub.4 are utilized, in addition to using the characteristics of the usual diodes, and therefore, the discharging efficiency is somewhat higher than that of the protection circuit of FIG. 5.
That is, where punch-through transistors are used, and the generated electrostatic voltage is higher than the punch-through voltage of these transistors, the current levels of the diode D.sub.1, the parasitic diodes of the transistors M.sub.1, M.sub.2, and the punch-through region can be utilized. Further, these discharge paths are parallel, and therefore, the capability of withstanding the voltage due toe electrostatic charges is increased.
However, where electrostatic charges are supplied between the power terminal V.sub.DD and the ground terminal G, if the charges are positive, the diode D.sub.6 takes a forward bias state to smoothly discharge them, as in the case of FIG. 5. By contrast, if the charges are negative, the diode D.sub.6 is operated in the reverse region, and therefore, the diode D.sub.6 can be damaged, even under a low level of electrostatic voltage.
Further, where electrostatic charges are supplied between the input pin and another one of the input pins, between the output pin and another one of the output pins, or between the input and the output pins, the diode is operated in the reverse direction, and therefore, the chip can be damaged, even under a low level of electrostatic voltage.
Most tests for electrostatic charges use charge from the body of a worker. However, the above-described charges are low-level charges which can cause damage to chips, and therefore, enhanced protection of such a chip against damage is urgently required.